I have activated a unit but don't want to move or fire yet. I don't want to activate another, as I would lose all of my remaining APs.

I'd rather wait, or "stall". In the boardgame I think stalling costs you 1 AP. I have skimmed through the PC-game rulebook and found nothing about stalling, but I have tried and found you can stall for free if you press Space Bar.

Yes Cesar if you pass using classic AP mode right now you get to keep your AP's. Classic mode does not currently think in 'activated' unit sense yet. Under persistent there was no need for there to be an activated unit, so the code really needs a lot of rewriting.